Abstract

Efficient semitransparent organic photovoltaic (OPV) cells are presented in an inverted geometry employing ZnS/ Ag/ WO3 (ZAW) as a top anode and ITO/ Cs2CO3 as a bottom cathode. Upon identification of the light absorption that differs depending on the illumination direction, the degree of the absorption asymmetry is tuned by varying the ZAW structure to maximize the efficiency for one direction or to balance it for both directions. Power conversion efficiency close to that of conventional opaque OPV cells is demonstrated in semitransparent cells for the ITO side illumination by taking advantage of the internal reflection occurring at the organic/ZAW interface. Cells with efficiencies that are reduced but balanced for both illumination directions are also demonstrated.

Figures (8)

(a) Schematic device geometry of the proposed semitransparent organic photovoltaic (ST-OPV) cells. (b) Equivalent multilayer thin-film structure under Smith’s method used for modeling/analysis of the proposed ST-OPV cells based on transfer matrix formalism. Definitions for Rt and Tt are given for the case of ITO-side illumination, for example. See Ref. 16 and references therein for further details on optical modeling.

(a) Refractive index (n) and extinction coefficient (k) of P3HT:PCBM70 blend films used in this work. The blend film was treated as if it were a single layer in ellipsometric analysis. (b) Comparison of the transmittance of P3HT:PCBM70 films on a glass calculated with the (n,k) values in (a) with respect to the experimental values.

Experimental J-V characteristics of ST-OPV cells under study with dZnS of (a) 50nm and (b) 20nm. That of a control inverted OPV cell with the same device structure except for the opaque anode [ = WO3(13 nm)/ Al (70 nm)] is also shown for comparison (dashed line).

Experimental and simulated EQE spectra of ST-OPV cells under study with dZnS of (a) 50nm and (b) 20nm. That of a control inverted OPV cell with the same device structure except for the opaque anode [ = WO3(13 nm)/ Al (70 nm)] is also shown for comparison (grey hexagon). Note: EQE spectra were measured for a batch different from those used in Fig. 5. This batch of cells had a Jsc of 9.9 (Ref), 8.6 (50nm; ITO), 4.1 (50nm; ZAW), 7.4 (20nm; ITO), 5.4 (20nm; ZAW) mA/cm2, respectively, under the illumination from the solar simulator.

(a) Calculated internal reflectance (RZAW(int)) at the organic/ ZAW interface for ITO-side illumination. (b) Distribution of the squared magnitude of electric field (≡ |E(z)|2) within ST-OPV cells when dZnS is 20 nm or 50 nm in case of ITO-side (top figure) or ZAW-side (bottom figure) illumination. Values are normalized to the squared field strength of the incident light (≡ |E0|2).

Measured total transmittance (Tt) of the ST-OPV cells. Inset: the photographs of the fabricated devices. Top parts of the finger electrodes correspond to the active regions. Bottom parts have additional 70 nm-thick Al layers for stable electrical contact.